You can unlock various interviews and gallery items by completing different chapters of the game.

Item	How to unlock
Gallery Item 1	Complete the Werewolf chapter
Gallery Item 15	Complete the Werewolf chapter
Gallery Item 16	Complete the Sea Creature chapter
Gallery Item 17	Complete the Frankenstein chapter
Gallery Item 18	Complete the Voodoo Queen chapter
Gallery Item 19	Complete the Mummy chapter
Gallery Item 2	Complete the Sea Creature chapter
Gallery Item 20	Complete the Dracula chapter
Gallery Item 3	Complete the Frankenstein chapter
Gallery Item 4	Complete the Voodoo Queen chapter
Gallery Item 5	Complete the Mummy chapter
Gallery Item 6	Complete the Dracula chapter
Interview 1	Beat the game.
Interview 10	Complete the Dracula chapter
Interview 5	Complete the Werewolf chapter
Interview 6	Complete the Sea Creature chapter
Interview 7	Complete the Frankenstein chapter
Interview 8	Complete the Voodoo Queen chapter
Interview 9	Complete the Mummy chapter